Feb 4, 2010
Selective nanoparticle immobilization using an electron beam
The fascinating electronic, optical and mechanical properties of nanoparticles make them important building blocks of modern nanotechnology – for example, in single-electron transistors, memory storage devices, nanoscale photodetectors and plasmonic waveguides. A current challenge in the development of practical devices is the ability to position and reliably secure single or arrays of nanoparticles in the required location.
In recent work, reported in the jounal Nanotechnology, scientists have developed a new approach. The technique exploits the build-up of material that accompanies exposure to an electron beam – a process known as electron-beam induced deposition (EBID) – to secure nanoparticles in place.
In EBID, an incident energetic electron beam results in a flux of lower-energy secondary electrons passing back through the surface close to the point of beam entry, causing the dissociation of pre-adsorbed molecules and subsequently the attachment of non-volatile reaction products. Long known as a detrimental side-effect of imaging in a scanning electron microscope, EBID is now finding increasing application in the field of nanofabrication.
By depositing a monolayer of nanoparticles from solution and then individually targeting selected nanoparticles for exposure to a sharply focused stationary electron beam, it has now been shown that the resulting localized build-up of material can be used to immobilize individual nanoparticles. Following ultrasonic agitation only targeted nanoparticles remain on the surface, with all others, including adjacent nanoparticles, having being removed.
Computer simulations show how material builds up on surfaces beneath the nanoparticle near where the incident beam exits and then enters the substrate, and results in a cup of material that holds the nanoparticle in place. Upper surfaces of the nanoparticle are largely free of material, offering the prospect of unaffected nanoparticle properties. Material build-up directly affects the strength of adhesion to the surface, and can be controlled through electron dosage and beam energy.
By controlling the path of the electron beam it should be possible to pattern nanoparticle arrays – making spot-exposure EBID an attractive new technique for nanofabrication.
About the author
This work was performed in the Nanoscience Group in the Department of Physics at the University of Bath. The research team comprised: Dan Burbridge, who was recently awarded his PhD for research into applications of electron-beam-induced deposition; Dr Simon Crampin, a computational theoretician with interests in nanoscale physics and in electronic structure and dynamics; and Dr Sergey Gordeev, an experimental scientist interested in areas of nanofabrication and atomic force microscopy. Prof. Guillaume Viau of the Departement de Genie Physique at INSA de Toulouse was responsible for the synthesis and characterization of the nanoparticles used.